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Fish Kills

by James B. “Jim” Kea
Area Extension Forestry Agent – now retired
Thursday, February 9th, 2006

We often get calls this time of year about fish kills. Often, pond owners suspect pesticide from a neighbor’s field, but more than likely, fish die from lack of oxygen.

Oxygen in farm ponds comes from two sources. First, it is incorporated by direct contact with air at the pond surface. The greater the surface area of the pond, the greater the incorporation of oxygen. Agitating the water creates more surface area, allowing more oxygen to be taken in. The second source of oxygen is plants. There are several kinds of plants in the farm ponds. The most common oxygen-producers are phytoplankton, microscopic plants that make the water look green. Other plants that you can see with your eyes also produce some oxygen.

Fish and some other aquatic animals use oxygen, but plants which make oxygen also use up oxygen in two ways. As plant and animal matter decay in ponds, they use up oxygen in this process. Living plants also use oxygen from the water at night or on cloudy days when they cannot make it.

The production and use of oxygen can easily favor a depletion of oxygen in ponds. Particularly sensitive are ponds overcrowded with fish, drawn down, too weedy, overly fertilized, or recently treated to kill weeds. Storms or high winds can also create a condition call turn over. Oxygen poor water near the bottom of the pond is forced to the surface as surface water is pushed to one side of the pond. This current produces a temporary reduction in oxygen but can kill fish in overcrowded ponds. Hot weather also reduces the oxygen holding capacity of the pond and also speeds up decay of organic matter.

During periods of high risk, inspect your pond every morning just before daylight. Fish under stress can be seen at the surface gulping for oxygen. If disturbed, they dive, but immediately come back to the surface. If the oxygen is not low enough to kill the fish, they’ll surface in early morning and return to deeper water as the oxygen builds up during the day through photosynthesis.

If fish continue gulping for air, flush the pond with aerated water from a well or another pond. If an irrigation pump is available, pump the water from the upper two feet. Aim the exhaust as parallel to the bank as possible to create a circular current in the pond. A simple, quick cure is to back a tractor-powered rotary mower into the pond and stir the water with the blades. A boat motor can help in a small pond.

Adding six to eight pounds of potassium permanganate per acre foot of water and 50 to 100 pounds of super phosphate per surface acre usually helps. The potassium permanganate helps slow organic matter decay, and the super phosphate stimulates phytoplankton growth.

Contact your local Extension agent for weed control help or your fisheries biologist with the North Carolina Wildlife Commission.


Revised 2/16/2006.

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